Carbon and electrospun nanostructures

ABSTRACT

The present invention is directed to the production of nanostructures, e.g., single wall carbon nanotubes (“SWNT”) and/or multi-wall carbon nanotubes (“MWNT”), from solutions containing a polymer, such as polyacrylonitrile (PAN). In particular, the invention is directed to the production of nanostructures, for example, SWNT and/or MWNT, from mixtures, e.g., solutions, containing polyacrylonitrile, polyaniline emeraldine base (PANi) or a salt thereof, an iron salt, e.g., iron chloride, and a solvent. In one embodiment, a mixture containing polyacrylonitrile, polyaniline emeraldine base or a salt thereof, an iron salt, e.g., iron chloride, and a solvent is formed and the mixture is electrospun to form nanofibers. In another embodiment, the electrospun nanofibers are then oxidized, e.g., heated in air, and subsequently pyrolyzed to form carbon nanostructures.

RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.60/484,335, filed Jul. 2, 2003. The entire teachings of the aboveapplication are incorporated herein by reference.

GOVERNMENT SUPPORT

The invention was supported, in whole or in part, by grantMDA972-02-C-0029 from the Defense Advanced Research Projects Agency(DARPA). The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

Carbon nanostructures, for example, single wall carbon nanotubes(“SWNT”) and multi-wall carbon nanotubes (“MWNT”), have attractedsignificant interest in recent years for their potentially uniqueelectrical and mechanical properties. A major impediment to thedevelopment of materials based on carbon nanostructure technology,particularly SWNT and MWNT carbon nanotube technology, has been therelatively high cost and low production rates of carbon nanostructuressuch as SWNT and MWNT. Pure single wall carbon nanotubes areparticularly expensive and typically have low production rates.Generally, the current synthesis methods for nanotubes utilize theself-assembly of gas phase carbon precursors onto a growing carbonnanotube. The production of carbon nanotubes, e.g., SWNT and MWNT, insuch a manner appears to be limited by the availability of the carbonsupply at the growing end of the carbon nanofiber and the population ofthe growing fibers within a reactor. The equipment costs associated withcurrently used methods for carbon nanotube synthesis, particularly SWNTand MWNT synthesis, indicate that true volume production is impractical.

SUMMARY OF THE INVENTION

The present invention is directed to the production of carbon andelectrospun nanostructures, e.g., single wall carbon nanotubes (“SWNT”)and/or multi-wall carbon nanotubes (“MWNT”), from mixtures containing apolymer, such as a thermoset (e.g. polyacrylonitrile or polyimide). Inparticular, the invention is directed to the production ofnanostructures, for example, SWNT and/or MWNT, from mixtures, e.g.,solutions, containing, for example, polyacrylonitrile, polyimide,polyaniline emeraldine base (PANi) or a salt thereof, an iron salt,e.g., iron chloride, and a solvent. In one embodiment, a mixturecontaining polyacrylonitrile, polyaniline emeraldine base or a saltthereof, an iron salt, e.g., iron chloride, and a solvent is formed andthe mixture is electrospun to form nanofibers. In another embodiment,the electrospun nanofibers are then oxidized, e.g., heated in air, andsubsequently pyrolyzed to form carbon nanostructures, e.g., SWNT and/orMWNT.

The present invention also includes a nanofiber produced byelectrospinning a polymer solution. For example, the invention alsoincludes a nanofiber comprising: (a) about 87 to about 99 weight percentof a thermoset polymer, such as a polyacrylonitrile or polyimide. Thenanofiber can include greater than zero and less than about 7 weightpercent polyaniline emeraldine base or a salt thereof; greater than zeroand less than about 5 weight percent iron or a salt thereof. Alsoincluded in the invention is a uniform carbon nanostructure compositionthat can include, for example, single wall carbon nanotubes and/or amulti-wall carbon nanotubes, produced by a method comprising the steps:(a) forming a polymer solution including a polymer and a solvent; (b)electrospinning the polymer solution to form nanofibers; and (c)oxidizing, e.g., heating in air, and pyrolyzing the fibers to formcarbon nanostructures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a Field Emission Scanning Electron Microscope (FESEM) image(50,000×) 15 nm polyacrylonitrile precursors to multi-wall nanotubesformed by the method of the invention.

FIG. 2 is a Transmission Electron Microscope (TEM) image (200,000×) ofelectrospun carbon fibers produced in accordance with the presentinvention. The marked tube in the image is about 15 nanometers indiameter.

FIG. 3 is a TEM image (200,000×) of electrospun carbon fibers producedin accordance with the present invention. A dark catalyst particle isshown in the center of the image.

FIG. 4 is a TEM image (500,000×) of a single wall bundle made from anelectrospun polyacrylonitrile by the method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of embodiments of the invention. All percentages and partsare by weight unless otherwise indicated.

The present invention is directed to the production of nanostructures,e.g., single wall carbon nanotubes (“SWNT”) and/or multi-wall carbonnanotubes (“MWNT”), from a polymer solution that includes, or example, adissolved thermoset polymer, such as polyacrylonitrile or polyimide. Inparticular, the invention is directed to the production ofnanostructures, for example, SWNT and/or MWNT, from mixtures, e.g.,solutions, containing, for example, polyacrylonitrile, polyimide,polyaniline emeraldine base (“PANi”) or a salt thereof, an iron salt,e.g., iron chloride, and a solvent.

In one embodiment, the mixture from which the nanofibers are electrospuncomprises polyacrylonitrile and a solvent that dissolvespolyacrylonitrile, for example, N,N-dimethylformamide (“DMF”). Themixture also contains polyaniline emeraldine base or a salt thereof andan iron salt. For example, the mixture contains about 1 to about 10,about 2 to about 5, about 3 to about 5, or about 4 weight percentpolyacrylonitrile. The mixture also contains at least some but less thanabout 2 weight percent polyaniline emeraldine base or a salt thereof,e.g., more than zero but less than about 1, about 0.1 to about 0.5, orabout 0.4 weight percent polyaniline emeraldine base or a salt thereof.The mixture also contains at least some but less than about 1 weightpercent of an iron salt, e.g., greater than zero but less than about0.9, about 0.1 to about 0.8, about 0.2 to about 0.7, or about 0.5 weightpercent of an iron salt. After the components are combined, the mixtureis preferably thoroughly mixed, for example, the mixture can besonicated for a time sufficient to dissolve all of the components. Inone embodiment, the mixture is ultrasonicated for about 8 hours.

In one embodiment, polymer nanofibers are produced by electrospinning apolymer solution that includes a polymer and a solvent for the polymer,such as an organic solvent. Examples of suitable polymers includepolyacrylonitrile, polyimide, and polyaniline emeraldine base or a saltthereof. Examples of other components include a salt, such as a metalsalt. Preferred salts include an iron salt, such as iron chloride. Anexample of a suitable organic solvent is N,N-dimethylformamide.Electrospinning uses an electrical potential, e.g., a kilovoltpotential, to coerce the rapid ejection of nanofibers from a polymersolution in a nozzle. The methods of this invention form electrospunnanofibers by directing a polymer stream from a spinning source in anelectric field created by source and counter electrodes, whereby thepolymer is splayed to form electrospun nanofibers. The electrospunfibers are deposited onto a portion of a collecting surface between thesource and counter electrodes.

Electrospinning in accordance with the present invention includes use ofa spinning source in an electric field that is created by one or moresource and counter electrodes. Fiber-forming polymeric material from aspinning source is directed into the electric field. The spinningprocess is driven by the electrical forces, generally in the form offree charges on the surface or inside the polymeric material. Thespinning source has one or more orifices from which the polymericmaterial is ejected and can be oriented anywhere in space in or adjacentto the electric field. For example, the spinning source can be in theelectric field, above the electric field, below the electric field, orhorizontally adjacent to the electric field. The counter electrode(s) isa component or components toward which the stream or jet of polymericfluid is directed due the presence of concentrations or areas ofelectric charge on the counter electrode(s). A collecting surface isinterposed between said source and counter electrodes.

For example, in one embodiment, the polymer mixture is electrospun usinga nozzle to substrate separation of about 3 to about 20 centimeters, aelectrical potential of about 3 to about 40 kilovolts, direct current(DC), and a mixture flow rate of about 0.02 to about 2 milliliters/hour,to produce nanostructures, e.g., nanofibers.

Nanofibers produced from the above-identified polymer mixtures range insize from about 1 to about 100 nanometers in diameter. For example,nanofibers produced in accordance with the present invention can havediameters of about 1 to about 50, about 1 to about 25, about 1 to about15, about 1 to about 10, or less than about 10 nanometers.

In one embodiment, the nanofibers are converted to single wall carbonnanotubes (“SWNT”) and/or multi-wall carbon nanotubes (“MWNT”). Thepolymer chains of the nanofibers are oxidized to cyclize the polymerchains of the nanofiber. The cyclized polymer chains of the nanofiberare then graphitized to form SWNT and/or MWNT. The graphitization of thecyclized polymer chains of the nanofiber can be conducted in thepresence of a catalyst, for example, iron or an iron containing compoundor salt. For example, the present invention includes the conversion ofthe polymer nanofibers to SWNT and/or MWNT by heating the nanofibers inair and then pyrolyzing the nanofibers, for example, in an inertatmosphere, to produce the SWNT and/or MWNT carbon nanostructures. Inone preferred embodiment, the electrospun nanofibers have diameterssmall enough so that formation of SWNT, versus MWNT, is preferred.Without wishing to be held to any particular theory, it is believed thatnanofibers having relatively small diameters, for example, having adiameter less than about 10 nanometers, less than about 5 nanometers orabout 2 nanometers, favor the formation of SWNT during graphitization ofthe nanofibers.

In one embodiment, the nanofibers are thermally oxidized by heated thenanofibers in air, for example, at a temperature of about 300° C. toabout 350° C., e.g., about 310° C., for a time sufficient to cyclize thepolymer chains of the nanofiber, e.g., about 5 to about 60 minutes,about 10 to about 40 minutes, about 15 to about 30 minutes, or about 20minutes. In another embodiment, the oxidized, cyclized nanofibers arethen placed in a furnace such as, for example, a tube furnace with aoxygen-free nitrogen purge. In one embodiment, the oxidized, cyclizednanofibers are heated to a temperature to effect pyrolyzation, forexample, the oxidized, cyclized nanofibers are heated to about 900° C.to about 2,400° C., e.g., about 1,000° C. to about 2,000° C., about1,100° C. to about 1,600° C., about 1,300° C. to about 1,500° C., orabout 1,400° C. In one embodiment, the rate of heating is about 10°C./min. Preferably, the oxidized, cyclized nanofibers are heated to asufficiently high temperature for a period of time sufficiently long toeffect graphitization of the polymer chains of the nanofiber. Forexample, the oxidized, cyclized nanofibers are heated to a pyrolyzationtemperature for about 1 minute to about 5 hours, about 1 minute to about1 hour, about 1 minute to about 30 minutes, or about 5 minutes. In oneembodiment, the nanofibers are heated in air at about 310° C. for about20 minutes and then placed in a tube furnace with a oxygen-free nitrogenpurge, heated to about 1,400° C. at about 10° C./min and pyrolized forabout 5 minutes at about 1,400° C.

Pyrolyzation of the nanotubes causes formation of a uniform carbonnanostructure composition. In one embodiment, a majority of the carbonnanostructures have a diameter within a range of about 20 nm of eachother. In a specific embodiment, at least about 50%, 60%, 70%, 80%, 90%or 95% of the carbon nanostructures of the composition having a diametergreater that 2 nanometers, have a diameter within about 20 nm of eachother. In another specific embodiment, the diameter of any of the samepercentages of carbon nanostructures of the composition are within about10 nm of each other. In still another embodiment, at least about 50%,60%, 70%, 80%, 90% or 95% of the carbon nanostructures will have adiameter in a range of between about 10 nm and about 20 nm.

EXAMPLE

A solution was formed in N,N-dimethylformamide of 4% by weight lowmolecular weight polyacrylonitrile (PAN) (Sigma-Aldrich Co.; MolecularWeight: 86,200 daltons), 0.4% by weight polyaniline (PANi) emeraldinebase (Organic Technologies, Lot# 17735NC), and 0.1% by weight ironchloride (FeCl₂). The solution was ultrasonicated until all of thepolyacrylonitrile had dissolved, typically about 8 hours. The solutionwas electrospun to form nanofibers using parallel plate geometry with a5 centimeter plate separation, a potential of 15 kilovolts (kV) DC, asolution flow rate of 0.25 milliliters/hour and a current of 160nanoamperes (nA). FIG. 1 is an FESEM of a 15 nm polyarcrylonitrileprecursor to a multi-wall nanotube formed by the method of theinvention. The nanofibers were then heated in air at 310° C. for 20minutes and then placed in a tube furnace with a oxygen-free nitrogenpurge, heated to 1,400° C. at 10° C./min and pyrolized for 5 minutes at1,400° C. The furnace was then cooled and the resulting carbonnanostructures were removed. Both the electrospun nanofibers and thecarbon nanostructures had diameters of about 10 to about 12 nanometerswhen measured from electron micrographs. FIGS. 2, 3 and 4 showTransmission Electron Microscopy (TEM) images of electrospun carbonfibers produced in accordance with the present invention.

Equivalents

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. A method for producing carbon nanostructures, comprising the stepsof: (a) forming a polymer solution including a polymer component and anorganic solvent; (b) electrospinning the polymer solution to formnanofibers; and (c) heating and pyrolyzing the fibers to form carbonnanostructures.
 2. The method of claim 1, wherein the carbonnanostructure formed is a single wall nanotube.
 3. The method of claim1, wherein the carbon nanostructure formed is a multi-wall nanotube. 4.The method of claim 1, wherein the polymer is a thermoset.
 5. The methodof claim 1, wherein the polymer includes at least one ofpolyacrylonitrile and polyimide.
 6. The method of claim 5 wherein thepolymer solution includes between about 1 and about 10 weight percentpolyacrylonitrile.
 7. The method of claim 6 wherein the polymer solutionincludes between about 3 and about 5 weight percent polyacrylonitrile.8. The method of claim 1, wherein the polymer solution further includesa salt.
 9. The method of claim 8 wherein the salt is an iron salt. 10.The method of claim 9, wherein the iron salt includes iron chloride. 11.The method of claim 9 wherein the polymer solution includes greater thanzero and up to about 0.5 weight percent iron salt.
 12. The method ofclaim 11 wherein the polymer solution includes between about 0.05 andabout 0.15 weight percent an iron salt.
 13. The method of claim 1,wherein the polymer includes a conductive polymer.
 14. The method ofclaim 13, wherein the conductive polymer includes at least one ofpolyaniline and polyethylene dioxythiophene.
 15. The method of claim 14,wherein the polymer solution further includes a metal salt.
 16. Themethod of claim 15, wherein the metal salt is iron chloride.
 17. Themethod of claim 1 wherein the polymer solution includes greater thanzero and up to about 1 weight percent polyaniline emeraldine base or asalt thereof.
 18. The method of claim 17 wherein the polymer solutionincludes between about 0.3 and about 0.5 weight percent polyanilineemeraldine base or a salt thereof.
 19. The method of claim 1, whereinthe carbon nanostructures formed have a particle distribution wherein amajority of the particles have a diameter within a range of about 20 nmof each other.
 20. The method of claim 19, wherein at least 70 percentof the carbon nanostructures have a particle diameter within a range ofbetween about 10 nm and about 20 nm.
 21. A method for producingnanofibers, comprising the steps of: (a) forming a polymer solutionincluding a polymer and an organic solvent; and (b) electrospinning thepolymer solution to form nanofibers.
 22. The method of claim 21 whereinthe polymer solution includes about 1 to about 10 weight percentpolyacrylonitrile.
 23. The method of claim 21 wherein the polymersolution includes more than zero to about 1 weight percent polyanilineemeraldine base or a salt thereof.
 24. The method of claim 21 whereinthe polymer solution includes greater than zero and up to about 0.5weight percent an iron salt.
 25. A uniform carbon nanostructurecomposition produced by a method, comprising the steps of: (a) forming apolymer solution including a polymer and a solvent; (b) electrospinningthe polymer solution to form nanofibers; and (c) heating and pyrolyzingthe fibers to form carbon nanostructures.
 26. The carbon nanostructurecomposition of claim 25, wherein a majority of the nanostructures have adiameter within 20 nm of each other.
 27. The carbon nanostructurecomposition of claim 26, wherein at least 70% of the carbonnanostructures have a diameter in a range of between about 10 nm andabout 20 nm.
 28. The carbon nanostructure composition of claim 27,including single wall nanotubes.
 29. The carbon nanostructurecomposition of claim 27, including multi-wall nanotubes.
 30. The carbonnanostructure composition of claim 25, wherein the polymer is athermoset.
 31. The carbon nanostructure composition of claim 25, whereinthe polymer includes at least one of polyacrylonitrile and polyimide.32. The carbon nanostructure composition of claim 31 wherein the polymersolution includes between about 1 and about 10 weight percentpolyacrylonitrile.
 33. The carbon nanostructure composition of claim 31wherein the polymer solution includes between about 3 and about 5 weightpercent polyacrylonitrile.
 34. The carbon nanostructure composition ofclaim 25, wherein the polymer solution further includes a salt.
 35. Thecarbon nanostructure composition of claim 34 wherein the salt is an ironsalt.
 36. The carbon nanostructure composition of claim 35, wherein theiron salt includes iron chloride.
 37. The carbon nanostructurecomposition of claim 35 wherein the polymer solution includes greaterthan zero and up to about 0.5 weight percent iron salt.
 38. The carbonnanotube composition of claim 37 wherein the polymer solution includesbetween about 0.05 and about 0.15 weight percent an iron salt.
 39. Thecarbon nanotube composition of claim 25, wherein the polymer includes aconductive polymer.
 40. The carbon nanotube composition of claim 25,wherein the conductive polymer includes at least one of polyaniline andpolyethylene dioxythiophene.
 41. The carbon nanotube composition ofclaim 25, wherein the polymer solution further includes a metal salt.42. The carbon nanotube composition of claim 41, wherein the metal saltis iron chloride.
 43. The carbon nanotube composition of claim 25wherein the polymer solution includes greater than zero and up to about1 weight percent polyaniline emeraldine base or a salt thereof.
 44. Thecarbon nanotube composition of claim 43 wherein the polymer solutionincludes between about 0.3 and about 0.5 weight percent polyanilineemeraldine base or a salt thereof.
 45. The carbon nanostructurecomposition of claim 25 wherein the carbon nanostructures have anaverage diameter of less than about 20 nanometers.
 46. The carbonnanostructure composition of claim 25 wherein the carbon nanostructureshave an average diameter of less than about 10 nanometers.
 47. Ananofiber, comprising about 87 to about 99 weight percent of a thermosetpolymer.
 48. The nanofiber of claim 47, wherein the thermoset includesat least on of polyacrylonitrile and polyimide.
 49. The nanofiber ofclaim 48, further including greater than zero and less than about 7weight percent polyanaline.
 50. The nanofiber of claim 49, furtherincluding a greater than zero and less than about 5 weight percent ironor a salt thereof.
 51. The nanofiber of claim 47 wherein the nanofiberhas a diameter of less than about 20 nanometers.
 52. The nanofiber ofclaim 51 wherein the nanofiber has a diameter of less than about 10nanometers.
 53. The nanofiber of claim 47, wherein the nanofiber is amulti-wall nanofiber.
 54. The nanofiber of claim 53, wherein thediameter of the nanofiber is between about 5 nm and about 30 nm.
 55. Thenanofiber of claim 47, wherein the nanofiber is a single wall nanofiber.56. The nanofiber of claim 55, wherein the diameter of the nanofiber isless than about 5 nm.